Layer-Dependent Superconductivity in Iron-Based Superconductors CsCaFeAsF and CaKFeAs.

In the quasi-two-dimensional superconductor NbSe, the superconducting transition temperature () is layer-dependent, decreasing by about 60% in the monolayer limit. However, for the extremely anisotropic copper-based high- superconductor BiSrCaCuO (Bi-2212), the of the monolayer is almost identical with that of its bulk counterpart. To clarify the effect of dimensionality on superconductivity, here, we successfully fabricate ultrathin flakes of iron-based high- superconductors CsCaFeAsF and CaKFeAs. It is found that the of monolayer CsCaFeAsF (after tuning to the optimal doping by ionic liquid gating) is about 20% lower than that of the bulk crystal, while the of three-layer CaKFeAs decreases by 46%, showing a more pronounced dimensional effect than that of CsCaFeAsF. By carefully examining their anisotropy and the -axis coherence length, we reveal the general trend and empirical law of the layer-dependent superconductivity in these quasi-two-dimensional superconductors.